The present invention relates to the production of rolled aluminum products having improved properties. More particularly, the invention relates to the manufacture of aluminum sheet products having controlled microstructures, which exhibit improved strength and fatigue crack growth resistance. The sheet products are useful for aerospace applications such as aircraft fuselages, as well as other applications.
Aircraft components such as fuselages are typically fabricated from aluminum sheet products. Resistance to the growth of fatigue cracks in such aerospace products is very important. Better fatigue crack growth resistance means that cracks will grow slower, thus making aircraft safer because small cracks can be more readily detected before they achieve a critical size which could lead to a catastrophic failure. In addition, slow crack growth can have an economic benefit because longer inspection intervals may be used. U.S. Pat. No. 5,213,639 to Colvin et al. discloses aluminum alloy products useful for aircraft applications.
The present invention provides rolled aluminum sheet products having improved resistance to fatigue crack growth, as well as other advantageous properties including improved combinations of strength and fracture toughness.
Aluminum sheet products fabricated in accordance with the present invention exhibit improved resistance to the propagation of cracks. Aluminum alloy compositions and processing parameters are controlled in order to increase fatigue crack growth resistance. This resistance is a result of a highly anisotropic grain microstructure which forces cracks to experience a transgranular or an intergranular tortuous propagation path. The number of cycles required to propagate these tortuous cracks to a critical crack length is significantly greater than the number of cycles required to propagate a crack that follows a smooth intergranular or non-tortuous path.
In an embodiment of the invention, alloy compositions, thermo-mechanical and thermal practices are controlled in order to develop an unrecrystallized microstructure or a desired amount of recrystallization. The microstructures are controlled with the help of dispersoids or precipitates which are formed at intermediate processing steps, or precipitation treatments to yield obstacles for dislocation and grain boundary motion. The sheet products comprise elongated grains, which form a highly anisotropic microstructure.
In accordance with one embodiment, the anisotropic microstructure may be developed as a result of hot rolling and additional thermal practices. The hot rolling temperature is controlled in order to facilitate the desired type, volume fraction and. distribution of crystallographic texture. In one embodiment, a recovery anneal after hot rolling yields the desired anisotropic microstructure after final solution heat treating and optional stretching and tempering operations. Additional intermediate anneals may be used to control the driving force for recrystallization.
The compositions of the aluminum products are preferably selected in order to provide dispersoid forming alloying elements, which control recrystallization and recovery processes during production. In one embodiment, mixtures of alloying elements that form the coherent Cu3Au prototype structure (L12 in the structurebereight nomenclature) are preferred. Such elements include Zr, Hf and Sc. In addition, alloying elements that form incoherent dispersoids such as Cr, V, Mn, Ni and Fe may also be utilized. Combinations of such alloying elements may be used.
An aspect of the present invention is to provide a rolled aluminum alloy sheet product having high levels of crystallographic anisotropy.
Another aspect of the present invention is to provide an Alxe2x80x94Cu base alloy sheet product having high levels of crystallographic anisotropy.
A further aspect of the present invention is to provide an aircraft fuselage sheet comprising a rolled aluminum alloy sheet product having an anisotropic microstructure.
Another aspect of the present invention is to provide a method of making an aluminum alloy sheet product having a highly anisotropic grain microstructure. The method includes the steps of providing an aluminum alloy, hot rolling the aluminum alloy to form a sheet, recovery/recrystallize annealing the hot rolled sheet, solution heat treating the annealed sheet, and recovering a sheet product having an anisotropic microstructure.
These and other aspects of the present invention will be more apparent from the following description.